In paperboard containers, a flat paperboard blank is folded over on itself to form a container that is square or rectangular in cross-section. The side ends of the container are sealed together to form the final structure. The paperboard is coated on its exterior and interior surfaces with a heat-sealable material that will bond to form the container. Typically, this heat sealable material is a low density polyethylene (LDPE) or other material having a melting point low enough to seal without damaging the paperboard.
There can be additional layers of material on the product side of the paperboard, between the paperboard and the interior sealing layer. One of these layers is usually a gas barrier layer. The gas barrier layer is either laminated, extruded, or co-extruded onto the paperboard to provide a board with gas barrier properties. Extrusion or co-extrusion is a faster, lower cost process than lamination. The cost of a laminate film and associated handling is also avoided with co-extrusion.
The need for a gas barrier in paperboard packages is well known and there have been many solutions to reduce gas transfer through the paperboard assembly. The amount and rate of gas transfer will depend on the type of barrier layer or layers that is used.
Particularly, the need for an oxygen barrier is well known. Oxygen contacting the product reduces the shelf life of the product. The amount and rate of oxygen transfer will depend on the type of barrier layer or layers that is used. The oxygen barrier layer is laminated, extruded or co-extruded onto the paperboard to provide a paperboard assembly with oxygen barrier properties. Oxygen is the gas for which barriers are tested but the barriers are useful for other gases so the barriers are commonly termed “gas barriers.”
There are other factors than oxygen permeability that enter into the choice of a barrier material. Cost is a factor, both in the type and cost of the barrier material being used and also in the amount required to obtain the necessary barrier properties. The number of layers or laminations of material are a factor in the cost.
The most effective, and most costly, oxygen barrier has been an aluminum foil barrier. Food, such as chips, has been placed in foil or plastic pouches. Aluminum foil has also been used in juice containers. Plastic materials, such as nylon, have also been used as barrier materials. Many other materials have been proposed and used in order to reduce the cost of the barrier material. Each of these materials has attributes and shortcomings. Usually, a number of layers of different materials are provided, with each layer having a special function. These functions might be the ease of attachment, a barrier for certain gases, a barrier for certain chemicals in the contained liquid, the ability to adhere two layers together, and the ability to seal the container. Each of these layers adds cost to the container, both in material cost and production cost.
An effective oxygen barrier will have an oxygen transfer rate (OTR) of 40 or less cubic centimeters of oxygen per square meter per day (cc/m.sup.2/day) at standard temperature and pressure (STP) and 50% relative humidity (% RH). Water soluble or water dispersible oxygen barrier materials such as polyvinyl alcohol require large quantities of material per square meter to be effective. This is usually more material than can be applied in a coating application such as a size press without manufacturing difficulty.
A paperboard packaging material minimizing oxygen permeability is desired.